1. Field of the Invention
The present invention relates to a sliding current collector such as a slip ring, commutator or the like which is made of a composite ceramics material and which is used in electric rotary machines such as commutator-type motors and generators. More particularly, the present invention is concerned with a sliding current collector which is made of a composite ceramics material and which is suitable for use in comparatively small electric motors such as an automotive engine starter motor, a motor for use in a portable motor-driven tool, and so forth.
2. Description of the Related Arts
Nowadays, various types of ceramics materials have been developed and used, including insulating ceramics which exhibit high insulation power well comparing with that of conventional insulators, as well as electrically conductive ceramics which exhibit electrical conductivity substantially the same as that of iron. Composite ceramics bodies also have been developed which are composed of highly insulating and highly conductive ceramics materials sintered into one body. Because of excellent mechanical strength and heat resistance, the composite ceramics bodies exhibit high resistances both to wear and sparking as compared with conventional materials such as carbon and copper and, hence, are expected to offer superior results when used as the material of a sliding current collector such as a slip ring or a commutator of an electric rotary machine.
An example of the sliding current collector made of a composite ceramics body is disclosed in Japanese Patent Laid-Open Publication No. 60-39338.
Unfortunately, however, no specific consideration or proposal has been given for the means for fixing a sliding current collector made of ceramics material to the shaft of an electric rotary machine. Conventionally, fixing of a sliding current collector to a shaft has relied upon press-fit or shrink fit. If such a conventional fixing method is used for fixing a sliding current collector made of a ceramics material, the collector becomes practically unusable due to cracking or destruction because of small tensile strength inherent to ceramics materials.
In order to prevent any excessive tensile stress from being generated in sliding current collector such as a slip ring or commutator ring made of ceramics material, it has been proposed to fix the collector by axially pressing it by means of a nut, while increasing the tolerance between the inside diameter of the slip ring or commutator ring and the outside diameter of the rotary shaft. This method, however, poses a new problem in that the axial pressing force for fixing the electrical collector is reduced due to axial elongation of the rotary shaft, with the result that the current collector is moved or offset on the rotary shaft, thus making it difficult to correctly position and hold the current collector.
These problems encountered with conventional arts will be described in more detail.
The use of a compound ceramics body as the material of a slip ring or a commutator ring of an electric rotary machine essentially requires means for correctly positioning the slip ring or the commutator ring on the rotary shaft of the machine both in radial and rotational directions, as well as means for preventing the ring from rotating relative to the rotary shaft.
In general, following methods have been used for the purpose of fixing a sliding current collector to a rotary shaft of an electric rotary machine:
(1) To fix the sliding current collector to the rotary shaft by means of a key.
(2) To decrease the tolerance between the sliding current collector and the rotary shaft to enable fixing by a shrink or press-fit.
(3) To press-fit the sliding current collector on knurled surface of the rotary shaft.
(4) To deposit a resin or the like material on and around the ends of the sliding current collector on the rotary shaft and to cure the same thereby fixing the collector.
As explained in, for example, ELEMENTARY CERAMIC SCIENCE, pp 113-142, published by Agune Kabushiki Kaisha, the ceramics materials generally exhibit only low levels of tensile strength, although the levels of compression strength are considerably high. The conventional methods (1) to (3) mentioned above, therefore, are not suitably used for ceramics current collectors because such collectors are easily broken due to cracking caused by tensile stress generated in the ring-shaped sliding current collector.
The known method (4) mentioned above also is inadequate in that the sliding current collector cannot be precisely located and centered because it is fixed by means of the resin which is deposited to and around the ends of the current collector.
In order to obviate such problems inherent in the conventional method, it is necessary to employ a greater tolerance between the outside diameter of the rotary shaft and the inside diameter of the cylindrical ceramics sliding current collector so as to avoid any tensile stress which may otherwise be applied to the inner peripheral portion of the sliding current collector.
However, unduly large tolerance between the outside diameter of the rotary shaft and the inside diameter of the cylindrical ceramics sliding current collector undesirably allows the sliding current collector to play or rotate relative to the rotary shaft due to the resistance torque imposed on the collector, thus posing another problem.
In order to ensure that intended performance of the machine is obtained and that electrical connection between the commutator riser and the rotor core coils is facilitated, it is necessary that the coil grooves in the rotor core and the riser grooves in the commutator have to be located at the same angular position as viewed in the direction of rotation. This requirement, however, cannot be satisfactorily met by known methods (1) to (4) because of difficulty encountered in precisely locating and fixing the cylindrical ceramics sliding current collector. Namely, with these known methods, it is not easy to connect the coils of the rotor to the commutator risers. The same problem is encountered also by the aforesaid known method (4). It is therefore necessary to take a suitable measure for overcoming these problems of the known arts.